The Things They Carried "The Lives of the Dead"

In the chapter "The Lives of the Dead" O'Brien explains that stories can bring the dead back to life through the act of remembering. 

“They’re all dead. But in a story, which is a kind of dreaming, the dead sometimes smile and sit up and return to the world.”

This quote explains how the dead can be with us if we remember them and keep them in our hearts just like how O'Brian keeps his fallen friends in his heart and writes about them so he can remember his memories with them and keep them alive in his mind for longer.